Depends on what you want out of learning a new language. If you want to become more familiar with the fundamentals of computers and how they work, C is the best

Depends on what you want out of learning a new language. If you want to become more familiar with the fundamentals of computers and how they work, C is the best option; if you want to learn a new paradigm and way of thinking, I recommend Haskell; if you plan on doing any web stuff, JavaScript is pretty much a must; and if you want to be at the forefront of new language developments, go for one of the shiny new languages like Go or Rust (I'm currently taking this route).